The average rate on a 30-year fixed mortgage fell to 3.59% last week, according to Freddie Mac. That's a new low for 2016 and the lowest rate for mortgages since February last year.

Such a low rate will no doubt encourage more homeowners to refinance, giving a boon to mortgage lenders facing an uncertain housing market. But there are several reasons to doubt an upcoming refinance boomlet could turn into a full-fledged boom.

These rates are low, but they've been low for a while.

Early last year, rates unexpectedly fell sharply, hitting 3.59% on Feb. 5 before rising, according to Freddie. That led more than 1.2 million borrowers to refinance in the first half of the year, according to Black Knight Financial Services.

Could that happen again? Sure, but it seems unlikely. A rate of 3.59% seemed shockingly small the first time rates dipped so low after the crisis, but now borrowers might find them downright quaint. Consider: Between 2012 and the end of 2015, Freddie reported the average mortgage rate 209 times. In 40 of those reports, or about a fifth, rates were lower than they are now.

When rates fell this low in February 2015, the Federal Housing Administration had just cut the insurance fees it charges on low-down-payment loans by half a percentage point. That gave borrowers a bigger incentive to refinance, and without it, many fewer borrowers would have likely bothered. That kind of catalyst isn't there this time.

Serial refinancers are tapped out.

As mortgage rates kept falling during the housing bust, borrowers with good credit would pull the trigger on a refi, only to find a year or two later that they could save a lot of money by pulling the trigger again.

In 2015's boomlet, such borrowers were significant. In the first half of 2015, about 1.2 million homeowners refinanced, according to a report last week by Black Knight. About 26% of those borrowers, when they refinanced, had loans that were less than two years old.

Such borrowers tend to be mortgage-savvy, notes Black Knight's Ben Graboske. They keep a close eye on rates and pick up "free money" as soon as it makes financial sense.

So what will such borrowers, who likely still have a loan that's less than two years old, see today?

Generally, borrowers aren't in the money for a refinance unless they can get a break of one percentage point, more or less, depending on how long they plan to keep the mortgage. Let's assume it takes a rate drop of half a percentage point, at a bare minimum, to entice a serial refinancer to pull the trigger.

In the 104 weeks preceding February 2015, the 30-year fixed rate was above 4.09% nearly two-thirds of the time. That means a borrower paying close attention would likely find it beneficial to refinance, even setting aside those borrowers with an FHA loan that got the additional break.

The picture doesn't look so rosy this time. In last two years, rates have been above 4.09% only a quarter of the time. That could mean that the borrowers paying the closest attention to the recent dip aren't yet in the money.